The present invention generally relates to magnetic random access memory (MRAM) devices. More particularly, the present invention relates to ferromagnetic cladding for concentrating a magnetic field at a sense layer.
An MRAM device includes an array of memory cells. The typical magnetic memory cell includes a layer of magnetic film in which the magnetization is alterable and a layer of magnetic film in which the magnetization is fixed or xe2x80x9cpinnedxe2x80x9d in a particular direction. The magnetic film having alterable magnetization may be referred to as a data storage layer or sense layer and the magnetic film which is pinned may be referred to as a reference layer.
Conductive traces (commonly referred to as word lines and bit lines) are routed across the array of memory cells. Word lines extend along rows of memory cells, and bit lines extend along columns of memory cells. Because the word lines and bit lines operate in combination to switch the orientation of magnetization of the selected memory cell (i.e., to write the memory cell) the word lines and bit lines can be collectively referred to as write lines. Additionally, the write lines can also be used to read the logic values stored in the memory cell.
Located at each intersection of a word line and a bit line is a memory cell. Each memory cell stores a bit information as an orientation of a magnetization. Typically, the orientation of magnetization in the data storage layer aligns along an axis of the data storage layer that is commonly referred to as its easy axis. External magnetic fields are applied to flip the orientation of magnetization in the data storage layer along its easy axis to either a parallel or anti-parallel orientation with respect to the orientation of magnetization in the reference layer, depending on the desired logic state (i.e., xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d).
The orientation of magnetization of each memory cell will assume one of two stable orientations at any given time. These two stable orientations are referred to as parallel and anti-parallel, and represent logic values of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d. The orientation of magnetization of a selected memory cell may be changed by supplying current to a word line and a bit line which intersect at the selected memory cell. The currents create magnetic fields that, when combined, can switch the orientation of magnetization of the selected memory cell from parallel to anti-parallel or vice versa.
A selected magnetic memory cell is usually written by applying electrical currents to the particular word and bit lines that intersect the selected magnetic memory cell. An electrical current applied to the particular bit line generates a magnetic field substantially aligned along the easy axis of the selected magnetic memory cell. The magnetic field aligned to the easy axis may be referred to as a longitudinal write field. An electrical current applied to the particular word line usually generates a magnetic field substantially perpendicular to the easy axis of the selected magnetic memory cell.
Preferably, only the selected magnetic memory cell receives both the longitudinal and the perpendicular write fields. Other memory cells coupled to the particular word line preferably receive only the perpendicular write field. Other magnetic memory cells coupled to the bit line preferably receive only the longitudinal write field.
The magnitudes of the longitudinal and perpendicular write fields are usually selected to be high enough so that the chosen magnetic memory cell switches its logic state when subjected to both longitudinal and perpendicular fields, but low enough so that the other magnetic memory cells which are subject only to either the longitudinal or the perpendicular write fields do not switch. The undesirable switching of a magnetic memory cell that receives only the longitudinal or the perpendicular right field is commonly referred to as xe2x80x9chalf-selectxe2x80x9d switching.
Manufacturing variation among the magnetic memory cells often increase the likelihood of half-select switching. For example, manufacturing variations in the longitudinal or perpendicular dimensions or shapes of the memory cells may increase the likelihood of half-select switching. In addition, variation in thickness or the crystalline anisotropy of the data storage layers may increase the likelihood of half-select switching. Unfortunately, such manufacturing variations decrease the yield of manufacturing processes for magnetic memories and reduce the reliability of magnetic memories.
As with nearly every electronic device, it is desirable to reduce the size and increase the package density of MRAM devices. However, a number of factors influence the package density that can be achieved for an MRAM device. First, the size of the memory cells usually must decrease with increasing package densities. Unfortunately, reducing the size of the memory cell can result in an increase of the magnetic field that is required to switch the magnetic orientation of the memory cell.
A second factor influencing the package density of an MRAM device is the size of the write lines themselves. As with the memory cells, the dimensions of the write lines must typically decrease with increased package density. However, reducing the dimensions of the write lines results in a corresponding reduction in the current that can be carried by the write lines. A reduction in current carried by the write lines results in a weaker magnetic field at the selected memory cell and impedes the ability to write the memory cell.
A third factor which influences the package density of an MRAM device is the distance between a write line and an adjacent memory cell (e.g., a memory cell that is not the xe2x80x9cselectedxe2x80x9d memory cell between the intersecting word and bit lines). As the distance between the write lines and adjacent memory cells decreases, the possibility increases that the magnetic field produced by a write line may inadvertently and adversely affect the information stored in an adjacent memory cell.
The problems associated with increasing the package density of an MRAM device have been addressed by others. For example, U.S. Pat. No. 5,039,655 to Pisharody discloses the use of a superconducting material around the three sides of a write line that are not adjacent to a memory cell. The superconducting material effectively shunts the magnetic field created by the current in the write line and directs the magnetic field toward the magnetic storage material of the memory cell. Similarly, U.S. Pat. No. 5,956,267 to Hurst et al., discloses a word line structure and method of manufacture therefore which improves upon the structure and construction methods of Pisharody.
One limitation of Pisharody and Hurst et al. is that the flux concentration means taught therein are restricted to three of the four sides of the write conductor. The maximum write field for a given write current is thereby limited by the width of the write conductor. A write line construction that overcomes this limitation and permits the creation of a stronger magnetic write field for a given write line width and/or a given current would be desirable.
Another limitation of Pisharody and Hurst et al. is that the flux concentration means which are formed around the write lines permit the creation of both a longitudinal component of magnetic field in the sense layer of the memory cell, as well as a perpendicular component of the magnetic field. The perpendicular component of the magnetic field is at best wasted in that it is unable to contribute to the orientation of the sense layer of the memory cell, and is perhaps harmful in that the perpendicular field may adversely affect adjacent memory cells. Thus, it would be desirable in some instances to provide a write line structure in which the longitudinal field components are reinforced and the perpendicular field components are reduced or eliminated entirely.
The present invention provides a cladded write conductor for use in a magnetic random access memory device. The cladded write conductor of the present invention permits the creation of a stronger magnetic write field for a given write line width and/or a given current.
In one embodiment, the write line structure for a magnetic memory cell comprises a write conductor having a front surface facing the memory cell, a back surface and two sides surfaces. A cladding layer is disposed adjacent a portion of the front surface of the write conductor, with the cladding layer terminating at spaced first and second poles adjacent the front surface of the write conductor. A data storage layer is operatively positioned adjacent the cladding layer. The distance between the poles is less than the width of the write conductor. The width of the data storage layer may be greater than or less than the distance between the poles.